A tree branch covered in nitrogen-loving lichen. (Credit: Martin Hutton)Air pollution may seem like an urban problem, but it’s becoming an increasing concern in California’s national parks.

Pollution from cars and trucks blows into the Sierra Nevada mountains, where it can have a dramatic impact on the ecosystem. In Yosemite National Park, researchers are trying to gauge that impact by using an unexpected tool: a fungus called lichen.

Yosemite Valley is known for its granite landmarks: Half Dome, El Capitan and the sheer walls that surround the valley. But according to botanist Martin Hutton, the granite isn’t really visible.

“Basically what we’re looking at is lichens. We barely even see this rock. It’s all lichens.”

The southern walls of Yosemite Valley are covered in black crust. Last year, Hutton repelled hundreds of feet down the cliffs to survey the species living here. “All sorts of different colors. All sorts of different shapes. They’re really special. There are no trees up there. There’s no shade.” Hutton says there are more than 500 species of lichen in Yosemite and many grow where few other plants can.

Lichens Connected to the Air

Despite looking tough, lichens are some of the most sensitive organisms in the ecosystem. Hutton uses a fallen tree branch to point out the species living there. “I see really deep saturated orange and that is Caloplaca. And there’s just this beautiful just deep saturated yellow and that is the yellow of the Candelaira.”

The yellow lichen, Candelaira, is warning sign for Hutton. “If you were to go to place with very little air pollution, then you would not be seeing this many of these Candelaria species,” said Hutton.

While most plants get nutrients from the ground, lichens get much of what they need from the air. “They are basically directly connected to the atmosphere. They’re connected to all of it. They see all of it. It’s one of the reasons they’re so sensitive,” said Hutton.

Lichen are sensitive to changes in the air, especially from air pollution. That makes them an indicator of bigger ecosystem changes. Hutton and his team are taking lichen samples at 300 sites around the park and analyzing them to see what story they tell.

Measuring Pollution in the Ecosystem

Further into a nearby pine forest, Hutton and his team have set up funnels that collect air pollution samples. But it’s clear something else has gotten there first. Hutton’s equipment is strewn across the ground, the victim of a curious black bear.

“Yeah, basically a bear grabbed this funnel and plucked it off the stake. They just want to make sure that there’s no food associated with this plastic funnel,” said Hutton.

This is one of 12 sites where researchers are measuring a key ingredient of air pollution: nitrogen. Nitrogen oxides are produced by car and truck exhaust. In Yosemite, nitrogen pollution isn’t only from nearby cars. It also arrives from elsewhere in the state.

“We all have experienced the westerly winds that happen that blow stuff essentially from over the ocean, across the Central Valley and up into the mountains,” said Lee Tarnay, Air Resource Specialist at Yosemite National Park.

Air pollution from urban areas is blown into the Sierra Nevada mountains by those westerly winds. And the problem is: nitrogen pollution is sticky. “That gas likes to stick to pine needles and just about anything else. And these trees act as a giant collector for the gases that stream through the air,” said Tarnay.

When it rains, the nitrogen pollution is washed off the pine needles and deposited on the ground. As any backyard gardener knows: nitrogen is a fertilizer.

“All plants need nitrogen to grow. And some plants need a lot of nitrogen and some need only very little. And so in Yosemite, we already had enough nitrogen to begin with,” said Hutton. Sierra Nevada forests are adapted to low levels of nitrogen.

Impact of Nitrogen Pollution in the Ecosystem

“We’re worried that additional fertilizer in Yosemite could have effects that we might not anticipate. We think that the Yosemite is system as is it should be now. So we want to make sure that if there’s something harming or changing that balance, then we want to know that,” said Hutton.

Hutton says that balance is already under threat by invasive plants and many respond to higher nitrogen levels. Nitrogen can also encourage more ground plants to grow, a major concern in fire country. “If you increase the amount of nitrogen, you have plants that basically fill up the space in between these natural patches. And so that means that fires can spread a lot better.”

Reducing car traffic in Yosemite could help cut air pollution. It will also depend on regional air districts across California, several of which, like the San Joaquin Valley, exceed federal air pollution limits. Hutton says he’s hopeful that research in Yosemite will help them identify pollution hotspots and manage the changes in the field.

Nitrogen makes up over 75% of the air we breathe. Therefore, it would seem that the nitrogen that’s causing problems is somehow different. I’m sorry to say that this confusion sort of took my mind off the main thrust of the story. But is there in fact some other gaseous nitrogen compound that’s causing the problem?

Lauren Sommer

Hi Lawrence – Thanks for the comment. I mention in the post above that the nitrogen pollution is made up of nitrogen oxides, which are produced in car and trucks exhaust, as well as other sources that use combustion. But thanks for pointing out that nitrogen gas is part of the air we breathe and isn’t a pollutant in that case.

Emily Schrepf, NPCA

Thank you for writing about the important issue of air quality in our national parks! While many people believe they can escape the polluted air in urban areas with a visit to our nearby national parks, unfortunately that is not the case. Yosemite, Sequoia and Kings Canyon National Parks suffer from some of the worst air in the country. In addition to nitrogen pollution, ozone pollution is prevalent in the parks as well. In 2008 Yosemite exceeded national air quality standards for ozone 33 times. Ozone pollution causes damage to Sequoia’s namesake trees; baby sequoias are thought to be sensitive to injury from ozone and one study found that nearly 90 percent of Jeffrey pines in or near the Giant Forest show visible signs of ozone injury like browning leaves.
You are absolutely right that the San Joaquin Air Pollution Control District needs to do more to protect these natural resources as well as the health of the people in the Valley and parks. Stronger Rules around ozone and particulate matter are absolutely necessary and the U.S. EPA needs to insure that these are adopted as soon as possible.
Emily Schrepf
Program Manager
National Parks Conservation Association

Author

Lauren Sommer

Lauren is a radio reporter covering environment, water, and energy for KQED Science. As part of her day job, she has scaled Sierra Nevada peaks, run from charging elephant seals, and desperately tried to get her sea legs - all in pursuit of good radio. Her work has appeared on Marketplace, Living on Earth, Science Friday and NPR's Morning Edition and All Things Considered. You can find her on Twitter at @lesommer.

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